Optical elements have long been used to diffract light. A common diffraction grating will diffract light according to the well known formula n.lambda.=d sin .theta. where n is the order, .lambda. is the wavelength, d is the grating spacing and .theta. is the angle from normal to the grating. Light will continue on in the original direction into the O order, as well as all other orders. Considering only the first order, light will be diffracted into both the +1 and -1 orders. FIG. 1A shows the relationship of the incident light to the +1, 0 and -1 orders.
It is often desired to send light off only in one direction, say in the direction of the +1 order, as shown in FIG. 1B. If a diffraction grating is used to direct the light, some light will necessarily also be directed into the -1 order, an assumed undesired direction. Indeed, any recording medium which records only the amplitude of the diffraction grating will direct light into both a desired and an assumed undesired direction. The optical efficiency of such a device is less than one which directed light solely into the desired direction.
By may of example, this effect occurs in holograms recorded with ordinary film. Upon illumination the original image used to record the hologram is seen. Additionally, a second pseudoscopic image is also formed in the opposite direction and a zero order component is transmitted.
An optical element or grating which records only phase can direct light exclusively into the +1 order eliminating the -1 and 0 orders. All of the incident energy may be directed into the desired direction. Gratings in which the maximum diffracted intensity occurs at one of the nonzero orders are hereby defined as "blazed" gratings for this invention. It is known in the art that reflection diffraction gratings may be constructed by providing a sawtooth stepped reflection surface. Blazed transmission gratings may be made holographically, by using dichromated gelatin for the recording medium, or as a Fresnel lens made, for example, of plastic. The main disadvantage of these techniques is that the recording medium is permanently encoded. They are not programmable.
Programmable optical elements exist. One class of such elements are spatial light modulators (SLMs). Generally, however, SLMs record only amplitude. One particular type of SLM is a magneto-optic spatial light modulator (MOSLM). A MOSLM consists of a two dimensional mosaic of individual magneto-optic cells. Each cell can be magnetized either parallel or anti-parallel to the direction of light propagation. this magnetization state then rotates the plane of polarization of linearly polarized light either counterclockwise or clockwise. The amount of light which passes through a second polarizer will then depend on the orientation of a downstream polarizer.
The effects of the MOSLM are shown in FIG. 2. Polarized light 20 is incident upon a MOSLM 22. The magnetization of the MOSLM 22 will be one of two directions. A analyzer polarizer 24 is placed downstream of the MOSLM 22. If the analyzer polarizer 24 is oriented to block the light rotated to the OFF direction, then the +1, -1 and 0 orders will be transmitted. However, if the analyzer polarizer 24 is oriented normal to the bisector of the ON and OFF states, as shown in FIG. 2A, only a +1 and a -1 component is transmitted. In this way the incident polarized light 20 has been converted by the MOSLM 22 and filter 24 into a beam reflecting only phase information from the pattern on the MOSLM 22. If the amplitudes of the +1 and -1 orders are equal, the MOSLM becomes a binary phase only grating. In this way the 0 order may be eliminated. However, the assumed undesirable -1 order remains.